1. Field of the Invention
The present invention relates to the heating and chemical treatment of aqueous process fluids such as the aqueous mining fluid used in the Frasch process for mining sulfur. More particularly, the present invention relates to the heating and chemical treatment of such aqueous process fluids using a sulfur-containing fuel as the energy source.
2. Description of the Prior Art
The Frasch process for mining sulfur is well known to those skilled in the art, and a description of its operation may be found in the patent literature and in numerous chemistry books and encyclopedias including, for example, the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Vol. 19, pp. 337-348, John Wiley & Sons, Inc., 1969. In the Frasch process, a hot aqueous mining fluid, e.g., water, is used to melt the solid sulfur present in an underground sulfur-bearing formation by injecting the fluid, heated under pressure to around 325.degree. F., through the annulus formed by two concentric pipes and using compressed air to lift the molten sulfur to the surface through the inner pipe. The air is usually forced down through a small diameter pipe located within the described concentric arrangement.
Until recent years, the source of heat for the operation of a Frasch process sulfur mine has been the relatively abundant, low-cost supply of sulfur-free gas. However, as these reserves dwindle and gas supplies, when available, soar in price, it is becoming increasingly necessary to resort to the use of other fuels.
The use of sulfur-free natural gas as the fuel and source of heat in Frasch sulfur mining operations permitted attainment of relatively high overall plant efficiencies, due in part to the fact that even the heat in the effluent combustion gases from the steam-generating boilers could be reclaimed by the incoming cold aqueous mining fluid through intimate, direct contact of the fluid and combustion gases in heat exchange units appropriately labeled "flue gas heat reclaimers".
In addition to providing low-level heat to the incoming aqueous mining fluid, a consequence of which was to reduce the oxygen content of the fluid and render it less corrosive, the combustion gases also provided carbon dioxide, a portion of which dissolved in the fluid, lowering its pH and thereby lessening its tendency to lay down alkaline scale deposits in the subsequent high-temperature heating stages.
Now that natural gas is relatively unavailable to industrial operations, it is becoming necessary to resort to the use of other fuels such as oil or coal, both of which usually contain varying amounts of sulfur. If these materials are used as fuel in the boilers and the resultant combustion gases used in the usual economical manner, i.e., by passing them through flue gas heat reclaimers to scavenge the heat, the aqueous mining fluid undergoes reduction in dissolved oxygen content, picks up scale-mitigating carbon dioxide, and dissolves large amounts of sulfur dioxide which are present in the combustion gases as the result of combustion of the sulfur in the fuel. Dissolution of this sulfur dioxide results in two problems. First, this acid gas lowers the pH of the fluid (makes it more acidic), thereby increasing the fluids corrosivity towards metals in the system. Secondly, there is an increased tendency towards deposition of calcium sulfite scale because of this material's extremely low solubility in aqueous fluids. In order to circumvent these problems it is possible to employ a system whereby the heat in the SO.sub.2 -containing flue gases is transferred to the aqueous fluid by indirect heat exchangers ("economizers") so that a considerable proportion of the heat normally reclaimed in direct contact heat reclaimers is still attained.
Such a system has serious drawbacks, however, such as, for example:
1. In order to offset the lack of CO.sub.2 pickup by the aqueous fluid in the indirect heat exchange system, a mineral acid has to be added, at some cost, to adjust the pH of the fluid so as to prevent the alkaline scale deposition previously described;
2. Cooling of the flue gases in the economizers results in condensation of water vapor which dissolves sulfur dioxide and carbon dioxide and produces a very serious corrosion problem with respect to the materials of construction of the economizer (unless the economizer were constructed of extremely costly acid-resistant alloys) since heat transfer tubes of normal low-cost materials of construction cannot be protected by coatings, etc., and still provide the required heat exchange rates; and
3. The incoming cold aqueous mining fluid should receive prior treatment with an oxygen-scavenging chemical to prevent extreme corrosion in the economizer as well as in the heat exchangers subsequently employed to heat the water to mining temperatures. While the chemical reactions involved in the removal of dissolved oxygen are fairly rapid at elevated temperatures, at the ambient temperatures of this system the reaction rates are very slow, unless increased by the use of a costly catalyst in addition to the oxygen-scavenging chemical, for example, by the use of cobaltous sulfate as a catalyst to promote the reaction between dissolved oxygen and an oxygen scavenger such as sodium sulfite.